That's a very valid question. Sometimes they are and sometimes they are not. My N900 has traveled to the jungles of South America, Italy, Canada, and through the Rockie Mountains (no connectivity) in CO. It was not able to perform up to par. My UMPC Needed to come along.

Few reasons:

1) Lack of social networking capability - to arrange meetings and such I used Facebook (this was before a decent client was released) was not a pleasant experience on the N900. It was too expensive to make phone calls.

2) Lack of RJ45 jack - all the hotels I staid in throughout Italy primarily used RJ45 jacks.

3) Keeping updated with my friends in the states via Facebook - once again not possible.

4) Online banking, it needed to be done and doing it on the N900 over South America's WiFi was horrid.

I could go on.

For a 5 day trip in the states my iPhone servers me pretty well regardless of my WiFi situation. I am able to keep up with all my friends via every social media network there is (even the ones I don't use), keep up with news/current events (where as Maemo had just one AP News), handle my finances, shop around/navigate a foreign city using the many apps (Yelp, AroundMe, Waze, etc.) and Siri.

There's a phrase, "Where's the train?" when you look at someone who is rushing. Well many times I find myself in that situation only the train is not proverbial. I might be late for a meeting in a city I have never been to before, these are things that my N900 has failed on me before.

I would say it would be a replacement for a trip over seas but only with WiFi because data costs would be too expensive.

The things I would want to do, that requires a laptop/UMPC/dexktop would be to Photoshop some photos and review them before posting them on a larger screen before posting them, maybe fine tune videos before I upload them (I am just guessing because I don't have that much experience with video editing on the iPhone), there is no need to sync with iTunes since everything is synced with iCloud so I don't need/want to do that, maybe triple check documents I have written on Word on my PC just to make sure it came out alright since I am new to iWorks. But honestly, I think the most important thing would be to rest my eyes from staring at a 3.5" screen for so long and have that comfort of Windows 7 back with a full keyboard and mouse and my 67 tabs open.

To contrast: I couldn't upload my videos taken on my trips to YouTube or Facebook with my N900 on the go, I needed a computer to do that, I can't even remember posting photos to Facebook or not. I am also a horrible speller so spell check is a MUST for me (cannot live without).